This invention relates generally to pressurized nickel-hydrogen storage cells, and, more particularly, to the use of such cells in extended life and deep discharge applications.
Rechargeable cells or batteries are electrochemical devices for storing and retaining an electrical charge and later delivering that charge for useful power. A familiar example of the rechargeable cell is the lead-acid cell used in automobiles. Another type of cell having a greater storage capacity for its weight is the nickel oxide pressurized hydrogen cell, an important type of which is commonly called the nickel-hydrogen cell and is used in spacecraft applications.
The nickel-hydrogen cell includes a series of active plate sets which store a charge electrochemically and later deliver that charge as a useful current, packaged within a pressure vessel that contains the electrolyte, the plate sets, and the hydrogen gas that is an essential active component of the cell. A nickel-hydrogen storage cell delivers current at about 1.3 volts, and a number of the cells are usually connected in series to produce current at the voltage required by the systems of the spacecraft.
A nickel-hydrogen cell used in a satellite is periodically charged by electrical current produced by solar panels on the spacecraft when the satellite is in sunlight, and then later discharged to supply electrical power, when the spacecraft is in shadow or peak electrical power is demanded. A satellite is a low earth orbit may experience up to about 6,000 cycles from light to dark conditions per year, with a corresponding number of cycles of charging and discharging the cells. A typical accepted industry design objective is attaining satisfactory operation through 30,000 cycles of charging and discharging, corresponding to an operating life of 5 years for the satellite in low earth orbit, or more yearsin other orbits where fewer battery cycles are experienced annually.
As the nickel-hydrogen cell cycles thousands of times, the maximum charge that it will hold gradually decreases, apparently due to chemical changes that occur slowly in the nickel electrode. The rate of the gradual diminution in cell capacity may be reduced by decreasing the depth of discharge of the battery. The depth of discharge is the percentage of the total charge capacity that is used during the discharge portion of the cycle prior to recharging the cell.
Present, state of the art, nickel-hydrogen cells using a 31 percent potassium hydroxide electrolyte can be cycled 30,000 times without unacceptable loss in charge capacity, but the depth of discharge must be held to about 30 percent or less to do so. That is, only about 30 percent or less of the electrical energy stored in the cell can be discharged during the discharge cycle. Alternatively stated, 70 percent or more of the stored energy is unusable and unavailable. Regular discharging of a higher fraction of the stored energy (termed "deep discharge") would cause an accelerated subsequent reduction in the total cycling capacity of the cell, resulting in a reduction in its operating life.
This limitation on the cell operating procedures greatly and adversely influences the ratio of stored energy per pound of cell weight. The ratio of stored energy to weight is a key spacecraft design consideration. The weight of the cell must be lifted to orbit, and with present launch vehicles the cost of moving a pound from the surface of the earth to orbit is typically $20,000. The ratio of stored energy per pound of weight could be significantly increased if the allowable depth of discharge could be increased.
Accordingly, there exists a need for an improved method of storing and delivering energy in a spacecraft. The method must provide the necessary cycles of operation with an acceptably low reduction in storage capacity, and provide an increased allowable depth of discharge under these conditions. The present invention fulfills this need, and further provides related advantages.